Some display devices can switch between three-dimensional (3D) display and two-dimensional (2D) display with naked eyes without using any glasses. The display devices are configured to include, for example, a first liquid crystal display panel which performs image display and a second liquid crystal display panel. The second liquid crystal display panel is arranged on the side of the display screen (the side of the observer) of the first liquid crystal display panel, and forms a parallax barrier for causing incidence of light independently for the right and left eyes of the observer at the 3D display. In this liquid crystal display device which can switch between the 2D display and the 3D display, the alignment of liquid crystal molecules in the second liquid crystal display panel is controlled. By so doing, the refractive index in the second liquid crystal display panel is changed, there is formed a lens (lenticular lens or cylindrical lens array) area which is extended in a vertical direction of the display screen and parallel to a crosswise direction, and the light of pixels corresponding to the right and left eyes is delivered to the visual point of the observer.
In the 3D image display device using the liquid crystal display lens, when the visual line has been moved, the crosstalk occurs between pixels for left eye and pixels for right eye, causing a problem of deteriorating the 3D image. To solve this problem, Japanese Unexamined Patent Application Publication No. Hei 7-72445 discloses a configuration to detect a spatial position of the head part of the observer, and to change the lens characteristics based on the detected position information of the head part.
FIG. 10 is a schematic cross sectional view of a three-dimensional image display device using a liquid crystal lens 100. In FIG. 10, the liquid crystal lens 100 and a display device 200 are adhered to each other through an adhesive member 300. The adhesive member 300 is a transparent member, and is formed using, for example, a UV (ultraviolet) curing resin. The display device 200 is formed using a liquid crystal display panel, an organic EL display panel, or the like.
FIGS. 11A and 11B are plan views of a liquid crystal lens corresponding to a part B-B′ in FIG. 10. In FIGS. 11A and 11B, a first substrate 10 is covered with a first electrode 11, over the entire surface of the display region. A pectinate second electrode 21 is formed on a second substrate 20, and one ends of the second electrode 21 are connected each other through a bus electrode.
FIG. 12 is a cross sectional view illustrating a configuration of a conventional liquid crystal lens corresponding to a sectional part A-A′ of FIG. 11B. In FIG. 12, the first electrode 11 is formed in a flat solid manner inside the first substrate 10 as a transparent substrate, and a first alignment film 12 is formed on the first electrode 11. The stripe-shaped (pectinate) second electrode 21 is formed inside the second substrate 20 as a transparent substrate, and a second alignment film 22 is formed to cover the electrode 21. The alignment direction of the first alignment film 12 and the alignment direction of the second alignment film 22 are the same. The first substrate 10 and the second substrate 20 are preferably formed with a glass substrate, or may be formed with a transparent plastic substrate. A liquid crystal layer 60 is sandwiched between the first substrate 10 and the second substrate 20.
In FIG. 12, the liquid crystal has positive dielectric anisotropy. In the three-dimensional image display device using the liquid crystal lens, it is possible to perform three-dimensional image display upon application of a voltage between the first electrode 11 and the second electrode 21. It is possible to perform two-dimensional image display when no voltage is applied between the first electrode 11 and the second electrode 21.
FIG. 13 is a cross sectional view illustrating principles of forming a three-dimensional image using one liquid crystal lens. In FIG. 13, human eyes recognize an image formed in the display device 200 through liquid crystal lenses 101. In FIG. 13, an image for right eye is “R”, while an image for left eye is “L”. In FIG. 13, the pitch of the liquid crystal lenses 101 is “Q”, and the pixel pitch of the display device 200 is “P”. The distance (i.e., interocular distance) between the center of the human left eye and the center of the right eye is “B”. In general, the interocular distance “B” is assumed as 65 mm. The pitch “Q” of the liquid crystal lenses, the pixel pitch “P” of the display device, and the interocular distance “B” are in a relationship as follows (equation 1).
                              [                      Equation            ⁢                                                  ⁢            1                    ]                ⁢                                                                                      Q        =                              2            ⁢            P                                (                          1              +                              P                /                B                                      )                                              (        1        )            
FIGS. 14A, 14B, 14C are cross sectional views illustrating principles of a liquid crystal lens. When a voltage is applied between the first electrode 11 and the second electrode 21, the line of electric force F is generated, as illustrated in FIG. 14A. When no voltage is applied between the first electrode 11 and the second electrode 21, the liquid crystals are horizontally aligned, as illustrated in FIG. 14B. In the illustration of the present application, the pretilt angle is ignored for prevention of complexity.
When a voltage is applied between the first electrode 11 and the second electrode 21, as illustrated in FIG. 14C, liquid crystal molecules 61 rise above the second electrodes 21, and the liquid crystal molecules 61 between pectinate electrodes are horizontally aligned. This results in a distribution of the refractive index, and results in a refractive index distribution type (GRIN: Gradient Index) lens.
The general conventional liquid crystal lens is one as illustrated in FIG. 11A to FIG. 14C. As illustrated in FIG. 14C, in the configured liquid crystal lens, disclination 80 occurs at the upper part of the pectinate electrode. This results in a problem that incident light is scattered at the upper part of the electrode, and the crosstalk increases. In this case, the disclination 80 implies the line of discontinuity resulting in the arrangement of the liquid crystal molecules, while the crosstalk implies that the image for left eye and the image for right eye are not sufficiently separated. Note that, if the crosstalk is high, it is seen simply as a doubly reflected image, instead of a three-dimensional image.
On the contrary, as illustrated in FIG. 15A and FIG. 15B, the alignment of the liquid crystal molecules in the liquid crystal lens is TN (Twisted Nematic) alignment. A polarizing plate 13 is arranged on the side opposite to the arrangement side of the liquid crystal on the second substrate 20, thereby possibly shading the disclination part. Thus, the crosstalk can possibly be reduced. In this case, the TN is approximately 90 degree twisted alignment. That is, in FIG. 15A, the alignment direction of a non-illustrated first alignment film formed on the first substrate 10 is 90 degrees from the alignment direction of a non-illustrated second alignment film formed on the second substrate 20. This mechanism will hereinafter be described.
FIG. 15A illustrates a state where no voltage is applied between the first electrode 11 and the second electrode 21. At this time, the liquid crystal lens unit includes normally open type TN liquid crystals. Thus, the image from the display device is not subject to any effect. FIG. 15B illustrates a case where a voltage is applied between the first electrode 11 and the second electrode 21. The liquid crystals are aligned in a manner that the lens is formed between the pectinate electrode and the pectinate electrode, as the second electrodes 21. Above the second electrodes 21, the line of electric force F is in a perpendicular direction to the second electrodes 21. Thus, the liquid crystal molecules 61 are also perpendicular thereto. That is, in this part, the light from the display device is absorbed by the polarizing plate 13, and thus is not transmitted. That is, it is possible to prevent the crosstalk.
In FIGS. 15A and B, in the polarizing plate 13, it is preferred that the transmission axis be approximately 90 degrees with respect to the polarization direction of the emission from the display device. If the display device is a liquid crystal display device, the emitted light is polarized light. However, if the display device is an organic EL display device, it is necessary to attach a polarizing plate onto the surface of the organic EL display device.
FIGS. 16A and 16B are cross sectional views for explaining this state. FIG. 16A is a cross sectional view illustrating the relationship between the polarization direction of incident light, the polarization direction of emitted light, and the transmission axis of the first polarization plate 13, when no voltage is applied between the first electrode 11 and the second electrode 21. In FIG. 16A, in the case of a liquid crystal lens in TN alignment in its initial alignment, the incident polarized light optically rotates in a liquid crystal layer by 90 degrees, when no voltage is applied. Thus, if the emission polarization direction is an X-axis direction, the polarization direction will be a Y-axis direction at the emission. If the polarization transmission axis of the polarizing plate 13 is in the Y direction, the incident light is transmitted through it. Therefore, in the case of two-dimensional display in which no voltage is applied between the first electrode 11 and the second electrode 21, the liquid crystal lens does not have any effect on the emitted light from the display device.
When a voltage is applied to the TN-aligned liquid crystal lens, the liquid crystal molecules 61 are aligned as illustrated in FIG. 15B. As seen from FIG. 15B, the liquid crystal molecules 61 rise above the second electrodes 21, thus deteriorating the optical rotation. However, around the center between the second electrodes 21 as the pectinate electrodes, the alignment of the liquid crystal molecules 61 is hardly changed from its initial alignment, resulting in optical rotation and rotation of an incident light polarizing axis by 90 degrees. Therefore, light is shaded above the second electrodes 21, while light is transmitted between the second electrodes 21.
FIG. 16B illustrates the above-described contents based on the relationship between the polarizing axes of the incident light and the emitted light. That is, when a voltage is applied between the second electrodes, the polarizing axis of the emitted light right above the second electrodes is in a direction perpendicular to the direction of the polarizing axis of the emitted light between the second electrodes. The polarizing axis of the polarizing plate 13 is in a direction perpendicular to the polarizing axis of the emitted light right above the second electrodes. Thus, the light is not transmitted right above the second electrodes.
FIG. 17 illustrates an example of a transmittance distribution of the TN-aligned liquid crystal lens. In FIG. 17, the horizontal axis represents the position, while the vertical axis represents the transmittance distribution. In the ideal transmittance distribution of FIG. 17, the transmittance will substantially be zero, near the second electrodes 21. However, in the actual TN-aligned liquid crystal lens, a problem is that the transmittance between the liquid crystal lenses cannot sufficiently be lowered, in relation to the electrode width of the second electrodes, the gap between the liquid crystal cells or the like.
In the liquid crystal lens, to attain the lens effect at most, the refractive index distribution preferably becomes a quadratic curve. FIG. 18 is a schematic cross sectional view illustrating an aligned state of the liquid crystal molecules in a conventional liquid crystal lens cell, and also a graph of a refractive index distribution corresponding thereto. In the graph of FIG. 18, symbols C and E correspond to the end part of the liquid crystal lens, while a symbol D corresponds to the center of the liquid crystal lens.
In general liquid crystal lenses, a large phase difference is required to attain the lens effect. Thus, it is necessary to have a certain degree of great interval (i.e. the gap) between substrates of the liquid crystal cells. On the contrary, the interval between the electrodes of the second electrodes 21 is several ten to several hundred μm. It has been understood, based on the previous study, that if the ratio of the gap to the interval between the electrodes is approximately 1:5, a quadratic curve can be attained. Therefore, as illustrated in FIG. 18, if the ratio of the cell gap to the interval between the electrodes is quite far from 1:5, the transmittance distribution is not to have a quadratic curve, in the alignment of the liquid crystal molecules 61. FIG. 18 illustrates a homogeneous-type alignment. However, the same problem occurs in the TN alignment.
As illustrated in FIG. 18, in the specification, the case where a lens is formed using a pair of second electrodes is identified as a single electrode. That is, in a single electrode, it is difficult that the refractive index distribution has a quadratic curve. As illustrated in FIG. 19, instead of forming the liquid crystal lens using a pair of second electrodes, two or more second electrodes 21 are arranged in one lens, and the electric potential of the second electrodes 21 is changed. By so doing, it has been considered that the refractive index distribution becomes closer to the quadratic curve. In FIG. 19, the second electrodes 21 are elongated in a stripe pattern in a vertical direction of the document, and different voltages may be applied respectively to the second electrodes 21.
In FIG. 19, the largest voltage V1 is applied to the electrode formed at the end part of the lens. As being closer to the inside the lens, the voltage to be applied to the electrodes is gradually decreased. That is, V1>V2>V3>V4.
Even if two or more second electrodes 21 are arranged in one lens, it is difficult that the refractive index becomes closer to a quadratic curve, because the gap of the liquid crystal cells and the diameter of the lens are largely different. In this specification, a system in which two or more second electrodes are arranged in one lens is identified as a multi-electrode.